Known wind power electricity generating systems comprise a hub; a number of blades fitted to the hub; and an electric machine comprising a stator and a rotor.
In actual use of these wind power electricity generating systems, the wind blows on the blades to rotate the hub about the axis, and so transfer the kinetic energy of the wind to the hub; and rotation of the hub is transferred to the electric machine, in particular to the rotor which is connected to and rotates with the hub about the axis.
The hub, blades, and rotor define the rotary assembly.
In these known wind power electricity generating systems, the angular speed of the rotary assembly must be detected to control the wind power system. More specifically, the angular speed of the rotor must be detected to control an inverter coupled to the electric machine, and/or to control the pitch of the blades with respect to the wind, and/or to calculate the power coefficient of the system, and/or to monitor system operation and efficiency, and/or to keep within a maximum angular speed.
The angular speed detection device most commonly employed in wind power systems is an encoder, of which there are various known types. The most commonly used are incremental and absolute encoders, which comprise a photodetector or proximity sensor.
Known incremental and absolute encoders comprise a disk, the lateral face of which has at least one succession of holes arranged in at least one circle; and a device for detecting the holes. The disk is fixed to the rotary assembly, and the hole detecting device is fixed to the nacelle.
One known incremental encoder disk has at least one succession of equally spaced holes, and the hole detecting device comprises at least one proximity sensor alongside the disk, or at least one light source and at least one photodetector on either side of the disk.
As the disk rotates, the hole detecting device detects the holes and generates a signal indicating the angular distance travelled and the angular speed of the disk, and therefore of the rotary assembly.
Some known incremental encoders have at least two proximity sensors or at least two photodetectors, and holes arranged in at least two circles, and detect the rotation direction of the disk.
In known absolute encoders, on the other hand, the holes are arranged unevenly in a given configuration in at least two circles, and the hole detecting device comprises at least two photodetectors or at least two proximity sensors. Absolute encoders process the signals from the proximity sensors or photodetectors to determine angular position with respect to a fixed reference.
One problem of using such known encoders in direct-transmission wind power systems lies in the encoder requiring a large disk fixed to the rotary assembly.
In some known direct-transmission wind power systems, the rotor and hub are hollow, are connected directly to each other, and have inside diameters allowing access by workers to the inside for maintenance or inspection. In such cases, using an encoder calls for a disk fixed to the rotary assembly and large enough to permit easy access, which poses two problems: the weight of the disk itself, and the precision with which the holes are formed, which affects the accuracy with which angular speed is determined. Moreover, encoders are sensitive to vibration caused by the blades; and the holes are subject to clogging by dirt, thus impairing reliability of the hole detecting device.